As temperatures in parts of northern India plunge during winter, an app-based railway ticketing service has predicted that train delays in the current fog season would be the worst in the last four years.

"The National Train Delay Index as measured by RailYatri this fog season in the last 15 days of December 2017 stands at 87 minutes with the worst of the typical fog days still to arrive in the northern part of the country," app-based travel portal RailYatri said.

According to the portal, the train delay results were calculated through the live train tracking data accumulated by it as reported by travellers between December 15 and 31.

The average time for trains delayed in 2014-15 stood at 72 minutes, while it improved a bit in 2015-16 to 47 minutes. But in 2016-17, the average time shot up to 96 minutes, it said.

Foggy winter is a phenomenon typically attributed to the northern part of the country but the train delays get spread across the country due to its cascading effects, it said.

RailYatri said that the worst affected due to fog were Guwahati-New Delhi Poorvottar Sampark Kranti Express, which was delayed by over 52 hours, and was followed by Anand Vihar-Sealdah West Bengal Sampark Kranti Express, which was delayed by over 48 hours.